The present invention relates to devices and systems for analyzing aerosols, and more particularly to instruments used to classify particles or other elements suspended in the aerosols according to their electrical mobility, size or other chosen characteristics.
A variety of devices have been developed for analyzing aerosols, particularly as to the particles or droplets suspended in the gaseous media (usually air) of the aerosols. Among these are differential mobility analyzers and electrical precipitators, which distinguish suspended elements of aerosols based on their electrical mobility. Since electrical mobility is inversely related to the size of the particle or other element, the DMA and precipitator effectively distinguish suspended elements according to size.
Other devices distinguish among particles based on particle mass and shape, scattered light, or diffusion, rather than electrical mobility. These include impactors, cyclones, horizontal elutriators and centrifugal separators.
Recent technological advances and discoveries have generated a strong interest in analyzing extremely fine particles, i.e. particles with diameters in the nanometer range. Certain "nanophase" materials composed of nanometer particles have been found to possess enhanced mechanical, optical and electrical/magnetic properties desired for advanced engineering applications. Semiconductor fabrication requires the control of nanometer particles. With the feature sizes in state-of-the-art devices at or below 0.35 micrometers, the need for micro contamination control must take into account particles smaller than about 35 nanometers in diameter. In magnetic disk drives, the distance between the aerodynamically supported transducing head and rigid disk is being reduced into the submicron range to allow more dense storage of magnetic data. In analytical chemistry, the study of aerosols to detect macro molecules in the nanometer range is increasingly accepted. Nanometer particles are suspected carcinogens which can penetrate deeply into lungs and are difficult to remove by lung clearing mechanisms. Accordingly, these particles increasingly are the subject of industrial hygiene and epidemiology studies, and the need for nanometer aerosol filtration is receiving increased attention. Photochemical reactions monitored in air pollution studies often begin with nanometer particles. In nucleation and condensation processes, nanometer particles serve as the incipient nuclei. Accordingly, aerosol element sizing instruments are being evaluated for their capacity to distinguish particles and other elements in the nanometer size range.
The differential mobility analyzer, (DMA) has shown the greatest potential for measuring nanometer aerosols. However, at sizes below about 20 nanometers, presently available differential mobility analyzers experience a deterioration in size resolution and detection sensitivity due to particle Brownian motion. More particularly, diffusional losses in the aerosol transport passages reduce detection sensitivity, and a broadening of the transfer function due to diffusion becomes pronounced at sizes below ten nanometers, which reduces resolution.
Resolution also is limited by a mismatch in the aerosol and sheath flows at the entrance to the classifying region of the DMA. The flow mismatch causes undesirable recirculation near the entrance, and becomes worse when the DMA is operated at high flow ratios (sheath flow: aerosol flow).
Undesirable electrical field effects also have more of an impact on nanometer particles. At the entrance to the classifying region is a slit that is sufficiently wide to allow an electric field to penetrate into the aerosol passage upstream of the slit. Also, use of an insulator near the high voltage electrode leads to a surface electric field effect that causes aerosol loss and electric field fringing around the monodisperse exit.
Accordingly, it is an object of the present invention to provide a differential mobility analyzer or other aerosol analyzing instrument in which the time of transfer through the aerosol transport passages is considerably reduced, thereby minimizing diffusion.
Another object is to provide a system capable of segregating and analyzing suspended particles in aerosols, particularly elements with diameters below ten nanometers, with increased sensitivity and higher resolution.
A further object is to provide a differential mobility analyzer less susceptible to undesirable electric field effects.
Yet another object is to provide a more compact and efficient DMA design.